Environmental scanning electron microscope

ABSTRACT

The invention provides for a scanning electron or ion beam instrument capable of transferring the beam from a high vacuum chamber ( 8 ) into a high pressure chamber ( 5 ) via aperture ( 1 ) and aperture ( 2 ). The beam is deflected and scanned by coils ( 3 ) generally positioned between apertures ( 1 ) and ( 2 ). The amplitude of deflection of the beam over a specimen placed inside chamber ( 5 ) is substantially larger than the diameter of aperture ( 1 ). Leaking gas through aperture ( 1 ) is removed via port ( 7 ) by appropriate pumping apparatus. The size of aperture ( 1 ) is such that the pressure in chamber ( 6 ) combined with the supersonic jet and shock waves naturally forming therein do not result in catastrophic electron beam loss in chamber ( 6 ). The addition of appropriate detection means result in an instrument characterised by superior performance over prior art by way of better field of view at low magnification, better vacuum system and improved detection and imaging capabilities.

TECHNICAL FIELD

[0001] The present invention relates to the technical field of electronand ion microscopy as well as to electron and ion beam technologies ingeneral.

BACKGROUND ART

[0002] Electron microscopes in general and scanning electron microscopes(SEM) in particular use an electron beam probe to examine specimens. Theelectron beam requires a good vacuum where it is generated by anelectron gun and propagated through focussing lenses all the way to thespecimen. In addition, many of the detection means used to detect theemerging signals from the beam-specimen interaction also require avacuum condition in which the specimen is severely limited. In the past,this meant that only dehydrated specimens could be used. In addition,because the electron beam delivers an electric current, the specimenshould generally have a conductive surface to prevent accumulation ofcharge that hinders normal operation of the instrument. This meant thatgenerally an insulating surface could not be examined. However, the morerecent technology of an environmental scanning electron microscope(ESEM) has made it possible to examine specimens in a gaseousenvironment. The presence of a gaseous envelope around the specimen atsufficient pressure makes it possible to maintain moist conditions sothat hydrated specimens can be observed in their natural state. Also,the ionised gas dissipates the electron beam current away from thesurface of insulating specimens and, therefore, these specimens need nothave the pre-treatments conventionally used to render their surfaceconductive. Additionally, the gas is used as detection medium to detectthe signals generated in the gaseous envelope around the specimen. Suchsignals are usually the secondary electrons and the backscatteredelectrons from the specimen, which ionise the surrounding gas and becomedetected by appropriate means.

[0003] This prior art is comprehensively described by U.S. Pat. No.4,596,928, DE Patent No. 0 022 356 B1, U.S. Pat. No. 4,992,662, U.S.Pat. No. 4,785,182, U.S. Pat. No. 4,823,006, U.S. Pat. No. 4,897,545 andU.S. Pat. No. 5,945,672.

[0004] One basic feature of this technology is the use of at least twopressure limiting apertures (PLA) to separate the high vacuum of theelectron optics column (where the beam is generated and propagated) fromthe high pressure environment of the specimen chamber. The gas leakingthrough the first of the apertures, PLA1, is quickly pumped out of thesystem before any significant amount of gas leaks through the second ofthe apertures, PLA2, into the vacuum of the electron optics column.

[0005] Now, despite the many and important advantages created with theadvent of ESEM over SEM, this technology has its own limitations whichare amenable to improvement.

[0006] One particular limitation in the ESEM is with respect to thepermissible field of view described herein. In a SEM operating totallyin vacuum, the electron beam is focussed into a very small probe, whichis scanned in a raster form over the specimen area observed, in asimilar manner as in an ordinary television set (i.e. a cathode raytube—CRT). However, the scanned raster over the specimen is very smallin absolute terms, which is of the order of a few mm or much less asopposed to the large screen of an ordinary television set. The verysmall scanned raster is possible to obtain because of the extremely fineelectron probe used, which allows, for example, one thousand lines to bestacked in the raster. Through appropriate electronics, a synchronousraster in a CRT creates an image corresponding point by point to thevariation of signals emanating from the scanned specimen surface. Theratio of the size of the image on the CRT screen over the size ofspecimen raster defines the magnification. Thus, the maximummagnification corresponds to the minimum raster on the specimen possibleto scan without distortion, noise or blurring (defining the ultimateresolving power of the instrument). At the other extreme, the maximumscanned area over the specimen defines the largest possible field ofview (corresponding to the lowest magnification). The largest field ofview possible is a useful specification of a microscope, because it isusual to survey a large area before a particular feature of interest ismagnified. Now, the maximum field of view in an ESEM is limited by thesize of the first pressure limiting aperture used. Usually, a 0.5 mmdiameter first pressure limiting aperture is used and hence the field ofview is much smaller than in a SEM, where there is no pressure limitingaperture and the field of view is limited by other electron opticalconsiderations, but nevertheless being superior to that of an ESEM.

[0007] The limited field of view in an ESEM is a consequence of the useof two pressure limiting apertures in a position, relative to thescanning coils, that creates a collimating effect on the probe withinthe scanned cone.

[0008] Because this is an important limitation, attempts have beendisclosed in prior art to rectify the problem. U.S. Pat. No. 5,362,964describes two approaches. According to the first approach, light opticsis used to obtain a light image at low magnification with lenses andmirrors located below the first pressure limiting aperture. Withappropriate electronics controls, the operator switches between thelight image and electron image at different magnifications. Whilst thisprovides one solution to the problem, it is characterised byconsiderable cost, cumbersome usage and technological complexity.

[0009] According to the second approach of the same U.S. Pat. No.5,362,964, a third scan coil is positioned between the two pressurelimiting apertures to bend the electron beam below the first pressurelimiting aperture beyond the boundary of the first pressure limitingaperture. This should be a better solution than the first because of theapparent simplicity. However, the latter solution is difficult inreality to achieve, because the third scan coil has limited space tooperate between the two pressure limiting apertures, it requires a highcurrent and the resulting multi-deflection system is difficult tomaterialise. This prior art has not yet been implemented on anycommercial instrument yet. The lack of such an otherwise useful systemfrom all commercial ESEMs available to date is indicative of thedifficulties involved. Notwithstanding the reasons for such a lack, thepresent invention discloses a much simpler solution to remedy thelimited field of view of an ESEM.

[0010] Another device of U.S. Pat. No. 5,485,008 also aims at improvingthe limited field of view in an ESEM. This attempt further demonstratesthe need to overcome the disadvantage of field of view. However, aperson skilled in the art of ESEM, gas dynamics and electron opticsimmediately recognises that the device of this patent does not readilyachieve the aim either. The increase of the aperture size at the expenseof pumping volume and all other suggested improvements would come at ahigh cost, they would create other technical problems to the system, andno technical reports are known showing the feasibility, implementation,or use of this device. Notwithstanding the reasons for the lack of useof the device as described in this patent, it will be appreciated thatthe present invention discloses a radically different solution from thisdevice with regard to the remedy of the limited field of view of anESEM.

[0011] Another limitation of an ESEM is that it generally requires largepumping capacity between the apertures as the first pressure limitingaperture is increased in order to increase the field of view.

[0012] A further limitation of an ESEM is that it generally requireshigh accelerating voltage in order to avoid the large electron beamlosses taking place above the first pressure limiting aperture where asupersonic gas jet and shock waves inevitably form. A large firstpressure limiting aperture creates a catastrophic amount of electronscattering at low accelerating voltage.

[0013] There is also a class of SEM instruments, which allow lowpressure gas of the order of 100 Pa. This class of SEM is often referredto as low vacuum or variable pressure SEM. This is achieved by generallyproviding increased pumping capacity above the last aperture placed inthe electron optics column generally several millimetres above the endof the final lens. This last electron optical aperture is generally of asmall diameter to limit the amount of gas leaking into the electronoptics column, but the field of view is maintained generally largebecause the position of the aperture is the same as in a conventionalSEM. The placement of the last aperture several millimetres above thebottom of the final lens has been dictated by electron opticalconsiderations of the conventional SEM. This location of the pressurelimiting aperture has prevented this class of SEMs to operate at asubstantially higher pressure as the ESEM does. In particular, the beamundergoes a catastrophic amount of losses in the travel distance betweenthe aperture and bottom of final lens and no useful beam spot can everreach a specimen placed even further below the final lens at substantialgas pressure as used in an ESEM. However, this class of SEMs has beenuseful in the elimination of charge accumulation on insulatingspecimens, as the generally low gas pressure achieved is sufficient forthis purpose. However, they cannot be used at substantial pressure or atlow accelerating voltage, which are desired features in the examinationof soft (i.e. low atomic number) or wet specimens.

[0014] The transition from the conventional SEM and low vacuum SEM,which are characterised by a large field of view at low magnifications,to all recent forms of ESEM has imposed certain limitations such as thelimited field of view and increased pumping capacity.

OBJECT OF THE INVENTION

[0015] It is an object of the present invention to provide an improveddevice to substantially overcome or ameliorate the above mentioneddisadvantages, and a further object to provide improved detection meansin the gaseous environment of the specimen chamber.

DISCLOSURE OF THE INVENTION

[0016] The embodiments of the present invention relate to environmentalscanning electron microscopes (ESEM) with improved field of view at lowmagnification, improved pumping capacity and improved detection in thegaseous environment.

[0017] In one aspect of the invention, there is disclosed a device usingelectron optical means for the generation and propagation of a focussedcharged particle beam transferred via two apertures from a high vacuumchamber into a gaseous specimen chamber with an intermediate pressurechamber therebetween, said apertures being aligned along axis of saidbeam, and characterised in that: (a) said beam is deflected by a set ofscanning coils positioned near axis of said beam; (b) all of said coilsbeing substantially between said apertures; (c) amplitude of thedeflection of said beam inside the specimen chamber is larger than thediameter of the aperture separating the specimen chamber from theintermediate chamber; (d) the aperture separating the specimen chamberfrom the intermediate pressure chamber is located substantially at theend of the electron optical means, or away from the end of the electronoptical means to allow a specimen to be freely placed withoutobstruction at any distance from the aperture. Pumping means attached tothe intermediate chamber allows operation at high specimen chamberpressure but such pumping means may be omitted for very low specimenchamber pressure work, for example, for suppression of specimencharging.

[0018] In the preferred embodiment, an improved environmental scanningelectron microscope (ESEM) has an enlarged field of view at lowmagnification in contrast to prior art. The prior art of ESEM uses twopressure limiting apertures (PLA) to restrict the flow of gas from thespecimen chamber into the vacuum of the electron optics column, wherethe electron beam is generated and propagated. A first pressure limitingaperture (PLA1) is usually placed near the end of the objective lens anda second pressure limiting aperture (PLA2) is placed near the principalplane of the same lens. The space between the two PLAs is pumped usuallywith a mechanical pump to accommodate the relatively large amount of gasflowing through the PLA1, so that only rough vacuum is maintainedtherein. The electron beam initially travels unobstructed in the vacuumof the electron optics, then it undergoes a limited number of scatteringevents in the rough vacuum of the space between the two PLAs and finallyenters in the high pressure regime of the specimen chamber. Care istaken for the beam not to travel too far before it reaches the specimen,so that enough un-scattered electrons remain in the original probe spotto carry out scanning and imaging of the observed area in the usualmanner. The shortest possible distance of beam travel in the highpressure chamber is achieved by placing the PLA1 at the end of the finallens. This is in contrast to the placing of a final aperture, as in thelow vacuum SEMs, several mm above the end of the final lens,substantially at the principal plane of the lens, which creates anobstruction between the specimen and the aperture, and which increasesthe travel distance in the gas from the final PLA to the specimen thuscreating a catastrophic scattering effect on the electron beam.

[0019] Any person skilled in the art of ESEM readily recognises that thedistance between the two PLAs must be kept as short as possible tominimise the beam losses in the rough vacuum conditions. Furthermore,any person skilled in the art of ESEM recognises that the distancebetween the two apertures may not become arbitrarily small because thegas can flow from the first aperture directly into the second one inlarge quantities, which destroys the good vacuum condition of theelectron optics column. Thus, a compromise distance between 6 and 15 mmhas been successfully used in practice.

[0020] Now, the electron beam undergoes deflection in two mutuallynormal directions to achieve the square raster scanned over the specimensurface. This is usually obtained via a set of double deflection scancoils along the optical axis of the system somewhere inside the final(objective) lens. Persons skilled in the art of electron optics arefamiliar with the details of this arrangement and it should beappreciated that the physical size of the scan coils does not allow themto be fitted between the two PLAs, but only above them as used in allprior art ESEM. The position of scan coils above both pressure limitingapertures is clearly shown in the cited prior art. As a result, theelectron beam is scanned with an effective “rocking” or “pivot” pointgenerally above said apertures, which limits the maximum amplitude ofoscillation within their diameter creating a collimating effect. Evenwhen the pivot point is located between the apertures, there again is acollimating effect of the two apertures. This collimating effect isresponsible for the familiar “tunnel vision in the ESEM” at the lowmagnifications resulting primarily by the fact that all scan coils areplaced above both said PLAs.

[0021] One remedy for this problem was described in U.S. Pat. No.5,362,964 by adding a third scan coil between the two PLAs, but thisimpractical approach is clearly set apart from the present disclosures.The placing of an additional scan coil between the two PLAs in an ESEMof the prior cited art has not in practice remedied the problemsassociated with the prior art.

[0022] Furthermore, the inability to place all coils between the twoPLAs in the prior art devices is due to the anticipated severe electronbeam losses in the rough vacuum of the relatively large distancerequired to accommodate all scan coils therein.

[0023] In one approach, the embodiments of the present inventionundertake the following steps to overcome the above limitations:Firstly, the scan coils are set and operate as in an SEM or low vacuumSEM but the PLA1 is placed at the end of the final lens and the PLA2 isplaced above all the coils used. In other words, the entire set of scancoils (one, two or three) is placed between the two PLAs, which isinitially contrary to established practice in ESEM. Now, to avoid theexcessive electron beam loss anticipated in the area between theapertures, a much smaller PLA1 is used, which allows a much smaller gasleak from the specimen chamber. Thus, without increasing the pumpingcapacity between the two PLAs, the vacuum will be higher as the leakthrough the PLA1 becomes smaller. Thus the extra length travelled by thebeam between the two PLAs is compensated by a proportionally bettervacuum, and hence the beam does not suffer additional losses. At firstsight, this might be thought to be an undesirable design because thePLA1 would be even more restricting the field of view on account of itssmaller size. However, the smaller diameter aperture actually iscompensated by eliminating the collimating effect of the double apertureas scan coils are situated between the apertures. The elimination ofcollimating allows an increased field of view without initially anymodification to the electron optics design of certain existinginstruments.

[0024] Furthermore, the elimination of the collimating effect allows theshifting of the pivot point so that the beam is allowed to scan an evenlarger base cone at the specimen. By such means, the scanned base of thecone is several times larger than the PLA1, thus achieving a large fieldof view, in effect, larger field of view than in prior art ESEMs.

[0025] The actual improvement of the field of view depends on thespecimen positioning below the PLA1. At the lower range of pressures,the specimen can be placed relatively far from the PLA and the field ofview is very large. As the pressure is raised in the specimen chamber,the specimen should be placed closer to the PLA1 with concomitantreduction of the field of view but nevertheless larger than the diameterof PLA1. Ultimately, at extremely high pressure the specimen should beso close to the PLA1 that the field of view will be again limited by thesize of the aperture. However, there exists a useful range of pressurethat allows a clearly larger field of view over prior art of ESEM forthe same range of pressure.

[0026] In another approach, the embodiments of the present inventionrely on the discovery of the relationship between (a) the amount ofelectron beam loss due to scattering by the supersonic gas jet and shockwaves naturally formed above the PLA1 and (b) the distance between thetwo PLAs. By use of a modern computer simulation technique of the gasflow, it has been found that the electron beam loss is constant as thedistance between the two apertures is varied, provided the gas exhaustsin vacuum. In other words, an increase of the distance between the twoapertures does not increase the electron beam loss, provided that theresidual back pressure due to the pump used is sufficiently low. Anyresidual back pressure due to the pump will add a fraction of electronloss in proportion to the distance and the residual pressure, asanticipated by prior art. This discovery now of the constancy ofelectron beam loss teaches that (a) the minimum distance between theapertures to be used is that which is required to allow convenientplacement of scan coils and (b) to choose the minimum pump speed thatdoes not add any significant electron loss due to back pressure abovethe constant amount of loss disclosed herewith.

[0027] Alternatively, the new discovery teaches that (a) the minimumdistance between the apertures to be used is that which is required toallow convenient placement of scan coils, (b) the minimum practicallypossible PLA1 is to be chosen that minimises the electron beam loss dueto the supersonic jet and shock waves and (c) the minimum pump speed isto be chosen so that the maximum intermediate pressure obtained combinedwith the supersonic speed and shock waves do not result in excessiveamount of electron beam loss that renders the probe spot unusable.

[0028] In practical terms, the new discovery and inventive steps of thepresent invention are materialised in the preferred embodiments by (a)placing scan coils between two pressure limiting apertures, (b) placingone of the two apertures at the end of the final lens or beyond, (c)making the distance of the two apertures as small as practical, (d)pumping the space between the two apertures with a pump that adds only avery low background gas pressure which added to the supersonic jet andshock waves naturally forming do not scatter the electron beam out ofits useful intensity, (e) choosing the size of the final aperture nottoo small as to interfere with the electron optics forming a usefulprobe and not too large as to create destructive supersonic jet andshock waves and (f) adjusting the electron optics components to operateat optimum efficiency.

[0029] The ESEMs of the preferred embodiments are generally capable tooperate at pressures sufficient to maintain saturation water vapourpressure in the specimen chamber, namely, pressures greater than 609 Pa.Therefore, pressures below 609 Pa are clearly also feasible in an ESEM,so that the embodiments of the present invention are also applicable inthe so called low vacuum or variable pressure region at any level belowthe threshold of 609 Pa.

[0030] Now, an ESEM is useful if it can sustain at least 609 Pa, whichis the minimum saturation vapour pressure of water in its liquid phaseat zero degrees Celsius. At higher or room temperature the saturationwater vapour pressure is higher. For a practical pressure range up to1000-2000 Pa, it can be shown that the pressure-distance relationship inthe specimen chamber is such that the field of view obtains asubstantial improvement with the present invention over and above thatsustained by the prior art of ESEM. Therefore, it should be appreciatedthat the present disclosure is in clear departure from the practice andunderstanding of prior art.

[0031] In yet another approach, and bearing in mind the disclosures madehereinabove, a further embodiment of the present invention undertakesthe following alternative steps to overcome the limitation of field ofview of prior art and to further increase the useful range of specimenchamber pressure: A third pressure limiting aperture (PLA3) ispositioned at a very short distance from the first aperture (PLA1),whilst scanning coils are placed between PLA3 and previous secondaperture (PLA2). The distance between the closely spaced apertures is ofthe order of magnitude of the PLA1. However, the diameter of the PLA3 issuitably smaller than that of PLA1 in order to avoid excessive amount ofgas ejected through it. The space between PLA1 and PLA3 is evacuated bypumping means. Because, the PLA3 is very close to the PLA1, there is nocollimating effect on the electron beam by the combination of first andthird aperture. With such condition, the scan coils can deflect theelectron beam via the PLA3 within a wide base cone allowed by the muchlarger PLA1 subtending a large solid angle at the PLA3. Thus, theinterplay of size and short distance of two PLAs placed on one side ofscan coils provides another improvement not practiced in the prior artof ESEM.

[0032] In an another aspect of the preferred embodiments of the presentinvention, the placement of two apertures at a very short distance asdisclosed hereinabove results in improvement of the pumping capacityrequired. Based on gas dynamical studies of the gas flow between twoapertures, it has been found that the velocity of the gas achieved afterit passes the PLA1 can be used to advantage. This high velocity gasdownstream of the PLA1 can be used to push back the background gastowards the input of the pump. As a result, a smaller capacity pump isrequired to create only a very rough vacuum. This advantage is onlyobtained when the distance is suitably short, generally less than a fewdiameters of PLA1. Clearly, this additional advantage was not disclosedor envisaged in the prior art, because the conditions used in prior artdid not pertain to this advantage of improved pumping.

[0033] The placement of the two apertures at very short distanceprovides an improved field of view over the prior art of ESEM withoutinitially any modification to certain existing designs of electronoptical systems. However, an even further improvement of the field ofview and improved overall performance is achieved by re-designing theelectron optics to shift the pivot point as close as possible to theclosely spaced apertures.

[0034] A further embodiment of the present invention anticipates avariable position of the pivot point above the PLA1 operating singly orin combination with PLA3.

[0035] The embodiments of the present invention do not initially requiremodifications to most existing electron optics systems but best resultsare achieved if those electron optics systems are adjusted to bestaccommodate the disclosures of the present invention. Adjustments may beeffected with regard to position and control of scanning coils and thethereby arising effective pivot point of the scanned beam. Adjustmentsmay be effected also with regard to position and control of probeforming apertures that determine the properties of the scanned probespot. Further adjustments may be effected with regard to incorporatingdifferential pumping inside the electron optics components in accordancewith the disclosures of the present invention. It will be appreciatedthat these and other adjustments of the electron optics systems for thepurpose of integrating the present invention with optimised electronoptics systems do not constitute departure from the spirit of thepresent invention.

[0036] In yet further embodiments of the invention, the improved ESEMemploys alternative means for signal detection in the gaseousenvironment of the specimen chamber. An electrostatic field is generatedabove the specimen surface under examination, so that an electronavalanche discharge ensues accompanied by a photon avalanche in the gasaccording to prior art. The electron and photon avalanches are initiatedby the slow secondary electrons (SE) and the fast backscatteredelectrons (BSE) emanating from the beam-specimen interaction. With asuitably shaped electrostatic field, the SE are mainly concentrated inan inner cylindrical portion of the gas around the optical axis, whilethe BSE act mainly in the annular portion of the gas surrounding theinner cylinder. Clearly, there is an overlapping annular portion betweenthe regions where there is a substantial mixture of the two signals. Therelative size and distribution of signals is a function of gas pressureand applied potential for a given gas. This description of signaldetection pertains to prior art.

[0037] Now, the embodiments of the present invention in relation tosignal detection consists of using fine optical fibres or light pipeswith a narrow or collimated input aperture in conjunction with a veryfine needle biased electrode, in order to separate the SE signal fromthe BSE signal in a satisfactory manner. The needle tip is generallythin and the electrostatic field has its maximum intensity only within afew diameters from the tip. The volume around the specimen is onlyweakly affected by this field, so that the detection volume thereby isused only to collect the ionising signals without initially creatingadditional ionisation from the applied field of the needle tip. When theelectrons enter in the strong field around the neighbourhood of the tip,they acquire enough energy to ionise the gas and multiply. Because ofthermal energy and diffusion, the electrons do not finish their travelonto the tip of the needle before they undergo a significant number ofrandom oscillations within the neighbourhood of the tip. Thus, everyelectron is multiplied accordingly within a small volume whereby thephoton avalanche also forms. This controlled and limited signal volumeis utilised by embodiments of this invention to both achieve high gainand separation of signals. The light so collected is used to analyse anddisplay the corresponding signals in the usual manner.

[0038] Increased efficiency of signal collection is achieved if morethan one needle electrodes are used, in order to capture those electronsthat can thermally diffuse away from the weak field without entering thestrong field around a single needle tip. Two or four such needlesinterconnected, or not, are generally sufficient to capture the majorityof SE and subsequently amplify them.

[0039] In other embodiments of this invention, the use of optical fibresis necessitated from the fact that as the pressure is raised in thespecimen chamber pressure, the range of SE is restricted to a smallerand smaller volume around the optical axis, whilst the action of the BSEremains expanded with an increased photon multiplication. Opticalfibres, thin light pipes, or collimated light collectors that limittheir photon input only from the desired excitation volume inside thegas are used. Collimation of the light pipes and selection of thedetection volume is achieved by known means of guard or shade walls atthe input of the light pipes.

[0040] In another embodiment of the present invention concerning signaldetection, a relatively large diameter light pipe is configured tocollect light from the volume of the gas excited only or mostly by theBSE signal. The BSE generate light in the gas through the dissipation oftheir own initial energy. In addition, the BSE generate further photonamplification via the avalanche process if an electric field is added inthe volume acted by the BSE. The light so collected is used to analyseand display the corresponding signals in the usual manner.

[0041] In yet another embodiment of the present invention concerningsignal detection, it is the electric pulse induced and picked up by theneedle itself that is used to display an image and to do analysis.

[0042] In a further embodiment of the present invention, multipleoptical fibres and light pipes in conjunction with electrically biasedelectrodes are used in the detection, amplification and separation ofsignals.

[0043] These and further objectives of the invention will becomeapparent from the following description of the preferred embodiments ofthe present invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a diagram of the relative position of scan coils betweentwo pressure limiting apertures as used in a device of a preferredembodiment of the present invention.

[0045]FIG. 2 is a diagram of the relative positions of scan coils andtwo closely placed pressure limiting apertures on one side of the coilsof another embodiment.

[0046]FIG. 3 is a diagram of a differential pumping system used inanother embodiment.

[0047]FIG. 4 is a diagram of a detection system with light pipes andneedle electrodes used in a further embodiment.

BEST MODES FOR CARRYING OUT THE INVENTION

[0048] To assist with understanding of the invention, reference will nowbe made to the accompanying drawings, which embody some examples of theinvention.

[0049] One embodiment of a device of the present invention is shown inFIG. 1. An electron or ion beam is generated, focussed and scanned by anelectron optics column by known means, the relevant parts only of whichare drawn in said FIG. 1. Apertures 1 and 2 are shown with scan coils 3being positioned between. The aperture 2 is the entry port for the beamfrom the upper electron optics column (not shown) containing anelectron/ion gun and other electromagnetic lenses, apertures and pumpingmeans according to known art. The beam travels through column liner pipe4 and exits via the aperture 1 into a specimen chamber 5. Anintermediate chamber 6 is located in the space between the apertures 1and 2 is evacuated via port 7 with a pump (not shown). The column liner4 freely communicates with the space in the intermediate chamber 6 atits low end and also at the top and/or middle part e.g. via holes (notillustrated). An evacuation path of the port 7 allows operation of thedevice at relatively high specimen chamber pressure but it is notnecessary at low specimen chamber pressure. The amount of pumpingdepends on the type of electron gun used and the level of high vacuumrequired thereto. The beam is deflected by the scan coils 3 in the x-ydirection, normal to the beam axis, along raster lines on the specimensurface so that the beam probes an area inside the specimen chamber 5much larger that the size of aperture 1. The size of probed (i.e.scanned) area over a specimen (not illustrated) depends on the effectiveposition of the pivot point of the used beam. The number of scan coils 3can be one, two or three in accordance with common practice but all ofthe scan coils are practically placed between the apertures 1 and 2. Theaperture 2 acts as a pressure limiting aperture to create a substantialpressure difference between the intermediate chamber 6 and the highvacuum chamber 8 where the beam is generated, but can also act as aspray or probe forming aperture according to known art. The aperture 1acts primarily as a pressure limiting aperture to create a substantialpressure difference between the intermediate chamber 6 and the specimenchamber 5, but can also act as spray or probe forming aperture. Pressuregreater than 609 Pa is allowed in the specimen chamber 5. A supersonicgas jet forms above aperture 1 whereby this supersonic gas jetdissipates its kinetic energy by collision with molecules of thebackground gas in the intermediate chamber 6 and by collision with wallsin the chamber 6. By result of the collisions, shock waves form in thebackground gas and in front of the walls in the intermediate chamber 6,the intensity of which depends on the pressure of the background gas, onthe distance of the encountered walls from aperture 1, on the diameterof the aperture 1 and on the pressure of gas in the specimen chamber 5.

[0050] Another embodiment of the invention is shown in FIG. 2. Thedescription is identical as per FIG. 1 with an additional aperture 9placed at close range from the aperture 1. The distance between theaperture 1 and the aperture 9 is only a few diameters of aperture 1,which is within the range of action of the supersonic jet formed by theaperture 1. This aperture positioning allows the beam to be deflected bythe scan coils 3 in the x-y direction so that the beam probes an areainside the specimen chamber 5 much larger that the size of the aperture1. The aperture 9 acts primarily as a molecular skimmer that allows onlythe central core fraction of the gaseous supersonic jet formed by theaperture 1 to escape into the intermediate chamber 6. The majority ofgas outside the central part of the jet forces itself into an evacuationchamber 10 wherefrom it is removed via port 11 by a pump (not shown).The configuration of FIG. 2 allows an even higher pressure in thespecimen chamber 5 than the configuration of FIG. 1.

[0051] One preferred embodiment of the pumping arrangement for thepresent invention is shown in FIG. 3. The port 11 is preferablyevacuated with a pressure differential or compression pump 12 such as amolecular drag pump. Advantage is taken of the fact that this type ofpump exhausts in the specimen chamber 5 so that the pump 12 must onlyovercome the pressure difference between the specimen chamber 5 and theevacuation chamber 10 instead of the difference between the evacuationchamber 10 and the atmosphere. This is a new improved approach becausethe pumping means acting on the intermediate chamber in the prior artexhaust in the atmosphere. It should be appreciated that the pump typeand configuration in FIG. 3 is optional and does not limit theobjectives and advantages of the invention disclosed in all otheraspects of the invention. However, this option constitutes an additionalimproved embodiment of the present invention. The specimen chamberpressure is maintained at any desired level via a regulating valvesystem 13 which opens or closes to admit gas in the chamber 5 or to pumpgas out of the chamber 5, in a manner that stabilises the pressure levelor varies the pressure to a different level.

[0052] Referring now to FIGS. 1, 2 and 3 a similar preferred embodimentof the pumping arrangement (not shown) is by incorporating a compressiontype pump with its input connected to port 7 to draw gas from thechamber 6 and its output connected to exhaust gas into specimen chamber5.

[0053] Referring again to FIG. 3, a similar preferred embodiment of thepumping arrangement (not shown) can be used to create a pressuredifference between the intermediate chamber 6 and chamber 10, by using adifferential or compression pump drawing gas from the intermediatechamber 6 and exhausting into the chamber 10.

[0054] Referring yet again to both FIGS. 1 and 2, similar compressiontype of pumps can be used between additional pumping chambers along theelectron optics column, namely, by connecting the pumping means betweenany two of the chambers 5, 10, 6, 8 and the additional chambers.

[0055] The idea of using compression type of pumps between stages asdisclosed herewith requires much smaller size pumps. The use ofminiature type pump has only become possible in the embodiments of thepresent invention, which teaches means for minimal pump requirements.The use of this type of pump has been excluded from all prior art inelectron microscopy because of the high pumping capacity generallyrequired in electron microscopy. Therefore, the incorporation of agenerally small size vacuum pump in the manner disclosed herewith is animprovement over known devices. Embodiments of this idea can beimplemented by various combinations and pump designs acting bycompression action between stages.

[0056] Referring yet again to FIG. 3, the pressure differential pump 12is a thermal transpiration-driven vacuum pump, which has some uniqueadvantages. The thermal transpiration-driven vacuum pump has nomechanical moving parts and hence has no mechanical vibration and noise,which generally deteriorate the image and resolution. This type of pumpcan be integrated with the instrument interfaces in a most effective,space and cost saving configuration. The use of this type of pump hasonly become possible in the present invention, which teaches means forminimal pump requirements. The use of this type of pump has beenexcluded from all prior art in electron microscopy because of the highpumping capacity generally required in electron microscopy. Therefore,the incorporation of a thermal transpiration-driven vacuum pump in themanner disclosed herewith is an improvement.

[0057] One preferred embodiment of the signal detection in the presentinvention is shown in FIG. 4. The pressure limiting aperture 1 isidentical as in previous drawings except for the detail whereby anaperture electrode 14 of the material at the aperture 1 is insulatedfrom the remainder of a wall 15 of the aperture 1 by insulating material16 which allows the application of an electric potential on the materialof the tip 14 independently of the potential applied on the material ofthe wall 15. The instrument beam (now illustrated) and referenced as 17passes through the aperture 1 and strikes the specimen 18 (nowillustrated) and referenced as 18 under examination inside a gaseousenvelope maintained by the specimen chamber 5. The beam 17 interactswith the specimen 18 and various signals emanate, in accordance withspecimen properties, from every pixel (point) element scanned by thebeam probe 17. The signals are low energy secondary electrons 19 andhigh energy backscattered electrons 20 and 21. The secondary electrons19 are generally confined in the inner portion of gas environment aroundthe incident beam 17 and are acted upon by the electric field generatedby the applied potential on a tip of a needle electrode 22. The electricfield first gathers the secondary electrons 19 towards the tip of theneedle electrode 22 followed by electron gaseous amplification in anavalanche form in the confined region surrounding the tip. The electronavalanche is accompanied by a photon avalanche from the excited andionised gas molecules, the light of which is collected by optical fibresor light pipe 23 directed towards the luminous avalanche region. Thelight pipe 23 is connected to light sensors (not shown) wherefrom theinformation about the specimen 18 is recorded according to known art.The potential of needle electrode 22 is generally higher than thepotential of aperture electrode 14 and of the potential of the specimen18 so that the secondary electrons 19 are directed towards the tip ofthe needle electrode 22. The potential of electrodes 14 and 22 isvariable and is chosen in accordance with the pressure in the specimenchamber 5 and in accordance with signal control desired by the operator.

[0058] Another preferred embodiment of the signal detection of thepresent invention is shown also in FIG. 4. Similar to the detection ofsecondary electrons 19 described hereinabove, the backscafteredelectrons 20 and 21 traverse a generally large volume of gas outside theregion of the volume occupied by the secondary electrons 19. Thebackscattered electrons 20 and 21 ionise the gas producing a newgeneration of secondary electrons from the gas molecules with which theycollide. This new generation of electrons are acted upon by the electricfield of electrode 24 and multiply in an avalanche form inside the gas,accompanied by a light avalanche in a fashion similar to that describedhereinabove. The light so produced is collected by a light pipe 25 orequivalent optical fibres which are connected to light sensors (notshown) wherefrom the information about the specimen is recordedaccording to known art. An electrode 26 is biased and shaped so that theelectric field separates and gathers the ionisation and photon productsof the backscattered electrons 20 and 21 towards the electrode 24. Thebackscattered electrons 21 also strike the electrode 26 and othersurrounding walls wherefrom yet another generation of secondaryelectrons is formed, known as “converted backscattered electrons” inprior art. The latter generation of secondary electrons are also actedupon by the field established by the electrodes 24 and 26, by thespecimen 18 and other surrounding walls, and are multiplied in anavalanche form to generate additional electrons 27, and photons alsocollected by the light pipe 25. The potential of electrodes 24 and 26 isvariable and is chosen in accordance with the pressure in the specimenchamber 5 and in accordance with signal control desired by the operator.

[0059] It should be appreciated that FIGS. 1, 2, 3 and 4 do not restrictthe scope and design of the present invention. Various parameters canvary to obtain a different information in different applications. Itwill be apparent to those skilled in the art that various changes andmodifications may be made therein without departing from the spirit ofthe invention.

[0060] The scan coils 3 shown in FIGS. 1, 2 and 3 are optimally designedaccording to known art in a fashion meeting the requirements andobjectives of the present invention. This invention does not restrictthe number and configuration of the scan coils 3. The coils 3 aredesigned and configured to occupy the shortest possible distance alongthe beam axis and generate an effective or apparent pivot point as closeas possible to the beam exiting aperture 1. A person skilled in the artchooses the appropriate number of turns, size, position and electriccurrent through said coils in conjunction with a given electron opticallens to achieve the set objectives and requirements.

[0061] It should be appreciated that to place the entire set of scancoils 3 between the two pressure limiting apertures 1 and 2 eliminatesthe collimating effect of previous art. This requirement should not beconstrued to mean that no part of any coil should be on the plane orpartly crossing the plane of any of the two apertures 1 and 2, providedalways that no collimating effect arises therefrom. Such variations, ifimplemented during the electron optical design, should by no means beinterpreted as departing from the spirit and scope of the presentinvention.

[0062] It should also be appreciated that the scope of the presentinvention is not limited by the particular type of electron optics usedto implement the teachings herewith. Generally speaking, the existingelectron optics designs can be integrated with the disclosures of thisinvention. However, better and improved results are obtained byre-designing the existing electron optics to optimally accommodate therequirements of the present invention.

[0063] One such improvement of electron optics is to shift the pivotpoint of the scanned beam as close as possible to the final pressurelimiting aperture.

[0064] Another improvement of the electron optics is to provide thefacility to vary the position of the pivot point along the axis of thesystem.

[0065] Any particular embodiment of the teachings of the presentinventions via various electron optical designs does not depart from thescope and spirit of the invention.

[0066] The column liner 4 shown in FIGS. 1, 2 and 3 should also have theshortest possible length and should have vents wherever possible so thatno pressure gradients are formed along its length. The column liner 4can also be eliminated wherever possible to allow optimum gasconductance, and exhaust through the port 7. These design details willensure minimal beam losses along the travel distance therein, the lossesbeing equal or close to the constant amount predicted by therelationship disclosed by the present invention.

[0067] The aperture 9 shown in FIG. 2 is preferably shaped to provide alip facing the incident supersonic jet issuing from the aperture 1. Thelip direction is generally along the direction of the flow lines of thegas jet issuing from the aperture 1 so that a shock wave is suppressedor prevented from forming in front of (i.e. upstream) the aperture 9.The latter feature is known in the art of skimmer design in molecularbeam technology and can be used to advantage in the ESEM of theembodiments of present invention. The elimination or suppression of ashock wave reduces the amount of gas leak through aperture 9. It shouldbe appreciated that these and similar improvements can be incorporatedin the arrangement of an ESEM herein disclosed without departing fromthe spirit of the invention.

[0068] Referring again to FIG. 4, the light pipe 25 is generally of amuch larger diameter than the optical fibres or light pipe 23, becausethe volume acted upon by the backscattered electrons is considerablylarger. However, bundles of optical fibres can be combined to achieve anequivalent result. The light pipes and optical fibres used arepreferably tilted as shown in the drawing in order to freely move andtilt the specimen in the chamber 5. The tilted light pipes alsofacilitate light collimation in order to more closely separate lightfrom different detection volumes in the gas. However, horizontaldirection of the light pipes may also be implemented without departingfrom the scope and spirit of the present invention.

[0069] It should be appreciated that more than one optical fibres andmore than one light pipes with corresponding biased electrodes can beused inside separate detection volumes in order to detect signalsemanating in different directions. Specifically with regard to thedetection of backscattered electrons, two or four light pipes can beused in pairs to detect backscattered electrons in all directions bycombining the output of said light pipes according to known art.However, any number of light pipes and any number of biased electrodescan be used to allow maximum flexibility regarding signal amplification,separation and filtration.

[0070] Referring yet again to FIG. 4, the various electrodes such as 14,22, 24, and 26 are connected via appropriate wiring 28, 29, 30 and 31 topower supplies and electronics amplifiers and controls according toknown art. The connections and cables are biased to shape the appliedpotential in the desired direction, to provide electrostatic andelectromagnetic guarding and to prevent “cross-talking” of theinformation from one channel to the next. The geometry of electrodesfurther allows shading, light collimation, shielding and separation ofone detection volume from another.

[0071] The embodiments disclose the use of a needle to generate electronamplification in the gaseous medium of an ESEM, so that the use of theelectric pulse in the needle alone can be used to display informationfrom the specimen. This mode of detection may be preferred in particularapplications.

[0072] The needle type of electrode can also be used inside a static oralternating magnetic field arising either from the focussing action oflenses or from the purposeful introduction of electromagnetic fieldsources to further control the electrons and ions generated inside thegaseous volume of the detector.

[0073] The embodiments of the present invention can also be used withother prior detection art such as with low bias electrodes not producingan avalanche type of signal amplification. It has been shown by priorart that it can be sufficient to collect the specimen secondaryelectrons and associated products even with a low bias electrode.Similarly, it has been shown that it is possible to collect theionisation current and associated products created by the backscatteredelectrons in the gaseous envelope surrounding the specimen underexamination by low bias electrodes.

[0074] Again, it will be apparent to those skilled in the art thatvarious changes and modifications may be made therein to allow acombination and integration with other instruments without departingfrom the spirit of the invention.

[0075] Industrial Applicability

[0076] The embodiments of the present invention apply generally to allinstruments using a charged particle beam such as an electron and ionbeam probe, which is generally scanned over a specimen surface. Suchinstruments comprise scanning electron microscopes, scanningtransmission electron microscopes, electron beam micoanalysers,environmental scanning electron microscopes and ion/electron beaminstruments as used in microelectronics industry for microfabrication.Electron beams are generated with different types of electron gun suchas tungsten, lanthanum hexaborite and field emission gun, so that any ofthese beam sources can be used without departing from the spirit of thepresent invention. Likewise, the focussing of the beam is achieved withdifferent type of electromagnetic lenses (electric or magnetic) so thatuse of any of such types will not be considered as a departure from thespirit of the present invention.

[0077] The applicability of the ESEM instrument is well establishedindustrially, and any improvement of it will also serve in same andadditional industrial applications.

[0078] The new ESEM disclosed by the present invention has the advantageof much improved field of view at low magnifications, which is a desiredproperty during the examination of specimens.

[0079] Another advantage is the minimal loss of beam during transferfrom the vacuum electron optics into the high pressure environment.Because a much smaller pressure limiting aperture is deliberately usedin the embodiments of the present invention, the jet formed above thepressure limiting aperture is significantly smaller and hencesignificantly less electron or ion beam losses occur in the region abovethe aperture as opposed to the situation in prior art which prefers theuse of large apertures. The lesser beam losses are also obtained by thebetter vacuum achieved along the electron beam path.

[0080] A further advantage is the improved signal-to-noise-ratio onaccount of better beam transfer conditions.

[0081] Another advantage is the higher pressure range that can betolerated in the specimen chamber, because of the better separation ofthe vacuum electron optics from the specimen chamber.

[0082] An additional advantage is the more efficient pumping meansdisclosed by this invention.

[0083] The overall advantage of the present invention is the use ofelectron optical means and small pressure limiting aperture tocompensate for a reduction of the required pumping capacity withconcomitant increase of the field of view over the prior art of ESEM andprior art of low vacuum or variable pressure SEM.

[0084] Other advantages arise from the improved detection meansdisclosed in the embodiments of the present invention. The use ofphotons instead of the electrons used in the prior art has the advantageof minimum noise. The photons collected from the gaseous phase can beefficiently further amplified by a photomultiplier which has a muchbetter time response than the electronics amplifiers used in prior art.A photomultiplier also requires simpler circuitry and can be moredirectly connected to the output display of the microscope.

[0085] A further advantage arises from the more efficient separation andamplification of signals in the gaseous medium of the specimen chamber.

[0086] A further advantage is with the use of the needle electrodeprinciple to multiply and detect the electrons in the gaseous phase ofthe specimen chamber when detection is preferred by the use of electroncurrent instead of photons.

1. A device using electron optical means for the generation andpropagation of a focussed charged particle beam transferred via twoapertures from a high vacuum chamber into a low gas pressure specimenchamber with an intermediate pressure chamber therebetween, saidapertures being aligned along axis of said beam, and characterised inthat: (a) said beam is deflected and scanned by a set of scanning coilssituated near said axis of said beam; (b) all of said coils beingsubstantially between said two apertures; (c) amplitude of thedeflection of said beam inside the specimen chamber is larger than thediameter of the aperture separating the specimen chamber from theintermediate pressure chamber; (d) the aperture separating the specimenchamber from the intermediate pressure chamber is located substantiallyat the end of the electron optical means, or away from the end of theelectron optical means to allow a specimen to be freely placed withoutobstruction at any distance from the aperture.
 2. A device usingelectron optical means for the generation and propagation of a focussedcharged particle beam transferred via two apertures from a high vacuumchamber into a high gas pressure specimen chamber with an intermediatepressure chamber therebetween, said apertures being aligned along axisof said beam, and characterised in that: (a) said beam is deflected andscanned by a set of scan coils situated near said axis of said beam andbeing substantially between said two apertures; (b) amplitude of thedeflection of said beam inside the specimen chamber is larger than thediameter of the aperture separating the specimen chamber from theintermediate pressure chamber; (c) the aperture separating the specimenchamber from the intermediate pressure chamber is located substantiallyat the end of the electron optical means, or away from the end of theelectron optical means to allow a specimen to be freely placed withoutobstruction at any distance from the aperture; (d) said intermediatepressure chamber is evacuated via port therein with an attached pumpmeans characterised by a minimum speed sufficient to suppress theintermediate pressure to a level whereby the gas molecules of backgroundgas together with gas molecules of a supersonic jet and shock waves,naturally forming between said two apertures, do not extinguish, byscattering, said beam out if its useful intensity; and (e) the apertureseparating the specimen chamber from the intermediate pressure chamberis sufficiently small to allow a minimum pump speed to suppress theintermediate pressure to a level whereby the gas molecules of backgroundgas together with gas molecules of a supersonic jet and shock waves,naturally forming between said two apertures, do not extinguish, byscattering, the said beam out if its useful intensity. between said twoapertures, do not extinguish, by scattering, the said beam out if itsuseful intensity; (f) the electron optics components are adjusted tooperate at optimum efficiency, said electron optics components includingbut not limited to location, type and configuration of detectors, scancoils, probe forming aperture, spray apertures, pressure limitingapertures, column liners, objective lens, electron beam and aperturealignment, electron optics controls, materials, electronics andsoftware.
 3. A device according to claim 2 wherein the aperture thatseparates the specimen chamber from intermediate pressure chamber isreplaced by a pair of apertures characterised in that: (a) the aperturesof the pair are substantially coaxial; (b) additional pumping meansevacuates the space formed between the said pair of apertures; (c)either aperture of the said pair of apertures together with the saidadditional pumping means may be removed.
 4. A device according to claim3 wherein either aperture of the said pair of apertures is moveablealong the common aperture axis by appropriate means.
 5. A deviceaccording to claim 3 wherein either aperture of the said pair ofapertures is moveable in direction normal to the aperture axis byappropriate means that allow precise alignment of the aperture along acommon axis.
 6. A device according to claim 2 wherein an additionalthird aperture is introduced between said two apertures andcharacterised in that: (a) said third aperture is located within therange of action of the said supersonic gas jet; (b) said third apertureskims central core of the supersonic jet formed by said first apertureso that only a small fraction of gas leaks into the intermediate chambercontaining said scan coils; and (c) said supersonic jet forces majorityof gas in the space between said first and third apertures wherefrom thegas is easier to remove by use of a small capacity pumping means.
 7. Adevice according to claim 2 wherein said pumping means comprises adifferential pressure pump exhausting into the specimen chamber.
 8. Adevice according to claim 3 whereby the pumping means have their inputand output connected between any two chambers.
 9. A device according toclaim 8 wherein any of said pumping means is a molecular drag pump. 10.A device according to claim 8 wherein any of said pumping means is athermal transpiration-driven vacuum pump.
 11. A device according toclaim 3 whereby any of said apertures acts also as a probe forming orspray aperture.
 12. A device according to claim 3 whereby effective orapparent pivot point of the scanned beam is located close to theaperture separating the specimen chamber from the intermediate pressurechamber.
 13. A device according to claim 3 whereby effective or apparentpivot point of the scanned beam is movable along the electron opticalaxis.
 14. A device according to claim 3 wherein detection means areintroduced comprising: (a) a needle electrode biased to attract theelectrons generated in the gas by the emerging ionising signals frombeam-specimen interaction; (b) said electrons multiplying in anavalanche form as approach said needle electrode; and (c) electric pulsebeing generated in said needle electrode by said multiplying electronsand recorded by appropriate means.
 15. A device according to claim 3wherein detection means are introduced comprising: (a) a needleelectrode biased to attract the electrons generated in the gas by theemerging ionising signals from the beam-specimen interaction; (b) saidelectrons multiplying in an avalanche form as approach said needleelectrode and generating a photon avalanche; and (c) photons in saidphoton avalanche being collected via optical fibres or light pipes andrecorded by appropriate means.
 16. A device according to claim 15wherein additional biased needles or other electrodes and additionaloptical fibres and light pipes are introduced to selectively detectvarious fractions of ionising signals generated from different regionsin the gas acted upon by different types of signals emanating from thebeam-specimen interaction
 17. A device according claim 3 whereindetection means are introduced comprising a needle electrode biased witha variable voltage to attract the electrons generated from thebeam-specimen interaction alone or in combination with ionizationelectrons from the gas, or to attract other ions from the gas ionised byvarious signals.
 18. A device according to claim 16 whereby said opticalfibres or light pipes collect photons corresponding to the secondaryelectrons emanating from the beam-specimen interaction.
 19. A deviceaccording to claim 16 whereby said optical fibres or light pipes collectphotons corresponding to the backscattered electrons emanating from thebeam-specimen interaction.
 20. A device according to claim 14 wherebyone of said ionising signals corresponds to secondary electrons from thebeam specimen interaction, and is used to make secondary electronimages.
 21. A device according to claim 14 whereby one of said ionisingsignals corresponds to backscattered electrons from the beam specimeninteraction, and is used to make backscattered electron images.